Catalysis of "outer-phase" oxygen atom exchange reactions by encapsulated "inner-phase" water in {V15Sb6}-type polyoxovanadates

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering

A novel mixed-valent hybrid chiral and polar compound, Fe₇As₃Se₁₂(en)₆(H₂O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe₅Se₉] chains connected via [As₃Se₂]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)₃]²⁺ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and ⁵⁷Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe²⁺/Fe³⁺ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by ⁵⁷Fe Mössbauer spectroscopy, showing that a fraction of the Fe³⁺/Fe²⁺ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe³⁺ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Composition-driven archetype dynamics in polyoxovanadates

Molecular metal oxides often adopt common structural frameworks (i.e. archetypes), many of them boasting impressive structural robustness and stability. However, the ability to adapt and to undergo transformations between different structural archetypes is a desirable material design feature offering applicability in different environments. Using systems thinking approach that integrates synthetic, analytical and computational techniques, we explore the transformations governing the chemistry of polyoxovanadates (POVs) constructed of arsenate and vanadate building units. The water-soluble salt of the low nuclearity polyanion [V₆As₈O₂₆]⁴⁻ can be effectively used for the synthesis of the larger spherical (i.e. kegginoidal) mixed-valent [V₁₂As₈O₄₀]⁴⁻ precipitate, while the novel [V₁₀As₁₂O₄₀]⁸⁻ POVs having tubular cyclic structures are another, well soluble product. Surprisingly, in contrast to the common observation that high-nuclearity polyoxometalate (POM) clusters are fragmented to form smaller moieties in solution, the low nuclearity [V₆As₈O₂₆]⁴⁻ anion is in situ transformed into the higher nuclearity cluster anions. The obtained products support a conceptually new model that is outlined in this article and that describes a continuous evolution between spherical and cyclic POV assemblies. This new model represents a milestone on the way to rational and designable POV self-assemblies.

Systems-based elucidation of the polyoxovanadate speciation reveals that heterogroup substitution can transform spherical kegginoids into tubular architectures in a programmable manner.

Three intersecting {V12} rings: {V30Sb8}, an ultra-large polyoxovanadate cluster shell

[VIV30SbIII8O78]12-, currently the largest known antimonato-polyoxovanadate (Sb-POV), features three perpendicular, intersecting 12-membered rings of edge-sharing O4V[double bond, length as m-dash]O square pyramids. While in two rings the apices of all O4V[double bond, length as m-dash]O pyramids point outwards, four apices of the third ring are directed into the cavity of the cluster shell, a concave structural motif not previously observed in polyoxovanadate chemistry. SbIII centers cap the eight niches defined by the octands of the {V30O78} cluster shell, resulting in discrete trigonal pyramidal SbO3 units, a second unprecedented feature. Within the resulting spin topology with numerous local geometrically frustrated motifs, the 30 spin-1/2 sites couple antiferromagnetically via a complex set of exchange pathways.

Nucleation mechanisms and speciation of metal oxide clusters

The self-assembly mechanisms of polyoxometalates (POMs) are still a matter of discussion owing to the difficult task of identifying all the chemical species and reactions involved. We present a new computational methodology that identifies the reaction mechanism for the formation of metal-oxide clusters and provides a speciation model from first-principles and in an automated manner. As a first example, we apply our method to the formation of octamolybdate. In our model, we include variables such as pH, temperature and ionic force because they have a determining effect on driving the reaction to a specific product. Making use of graphs, we set up and solved 2.8 × 105 multi-species chemical equilibrium (MSCE) non-linear equations and found which set of reactions fitted best with the experimental data available. The agreement between computed and experimental speciation diagrams is excellent. Furthermore, we discovered a strong linear dependence between DFT and empirical formation constants, which opens the door for a systematic rescaling.

New pronounced progress in the synthesis of group 5 polyoxometalates

This highlight summarizes new developments made in group 5 polyoxometalate science of high nuclearity clusters with focus on synthetic approaches.

Two organic–inorganic hybrid polyoxovanadates as reusable catalysts for Knoevenagel condensation

Two novel polyoxovanadates as heterogeneous catalysts have exhibited excellent catalytic properties in the Knoevenagel condensation, especially compound 1's activity is basically maintained after three cycles.

Soluble Hetero-Polyoxovanadates and Their Solution Chemistry Analyzed by Electrospray Ionization Mass Spectrometry

Polyoxometalates (POMs) are an intriguing class of compounds due to their tremendous structural variety and the wide spectrum of resulting properties, which make them interesting for applications in fields such as catalysis, material science or nanotechnology. Their ability to form large supramolecular architectures by self-assembly offers an entry to complex, functional systems. After an introduction into the structure and synthesis of POMs of the early transition metals, recently discovered water-soluble antimonato polyoxovanadates (Sb-POVs) and the investigation of their chemical reactivity are discussed. Electrospray ionization mass spectrometry (ESI-MS) is presented as an analytical technique suitable to investigate the structure of complex POM assemblies in solution and to probe the underlying reactivity and formation mechanisms. This Minireview highlights the first studies on the soluble Sb-POVs and how the knowledge of their reactivity obtained by ESI-MS has fostered the syntheses of numerous novel Sb-POV compounds.

Mixing SbIII and GeIV occupancy in the polyoxovanadate {V14E8} archetype

The first mixed germanato-antimonato derivative of the {V14E8O42} (E=semi-metal) cluster archetype has been synthesized under hydrothermal conditions and isolated as {Ni(phen)3}2[α-VIV 14SbIII 5GeIV 3O42(OH)3(H2O)]· ≈16H2O (phen=1,10-phenanthroline). In the cluster anion, seven of the eight hetero-metal positions are occupied by disordered Ge/Sb atoms, while one position is fully occupied by Sb atoms. The [V14Sb5Ge3O42(OH)3(H2O)]6− anions are arranged in pairs with remarkably short inter-cluster Sb···O contacts. Bond valence sum calculation strongly suggests that the Sb···O contact must be taken into account as a weak bond. The magnetic properties are dominated by strong intra-cluster antiferromagnetic exchange interactions, and the cluster anion is magnetically quasi-isolated from the spins of the Ni2+ complex cations.

Ordnung muss sein: heteroelement order and disorder in polyoxovanadates

Three new mixed antimonato–germanato polyoxovanadates exhibit different degrees of order of their unique crystallographic heterometal atom positions.

A multi-purpose reaction cell for the investigation of reactions under solvothermal conditions

A new versatile and easy-to-use remote-controlled reactor setup aimed at the analysis of chemical reactions under solvothermal conditions has been constructed. The reactor includes a heating system that can precisely control the temperature inside the reaction vessels in a range between ambient temperature and 180 °C. As reaction vessels, two sizes of commercially available borosilicate vessels (V_max = 5 and 11 ml) can be used. The setup furthermore includes the option of stirring and injecting of up to two liquid additives or one solid during the reaction to initiate very fast reactions, quench reactions, or alter chemical parameters. In addition to a detailed description of the general setup and its functionality, three examples of studies conducted using this setup are presented.

In situ ligand exchange-mediated 0D/1D transformation of a polyoxovanadate

The antimonato-polyoxovanadate {Ni^II(en)₃}₃[VIV15SbIII6O₄₂(H₂O)]·ca.15H₂O was utilized as a synthon for the solvothermal in situ generation of the new compound {Ni^II(phen)₃}₂[{Ni^II(en)₂}VIV15SbIII6O₄₂(H₂O)]·19H₂O, a rearrangement induced by ligand metathesis.

Immobilization of Keggin polyoxovanadoniobate in crystalline solids to produce effective heterogeneous catalysts towards selective oxidation of benzyl-alkanes

The catalytically active PNb₁₂O₄₀(VO)₂ was immobilized in crystalline solids and functioned as an effective robust catalyst towards selective oxidation of benzyl-alkanes.

An unprecedented antimonato-polyoxovanadate (SbPOV) based on both α-[V14Sb8O42]4− and β-[V14Sb8O42]4− isomers

An unprecedented antimonato-polyoxovanadate (SbPOV) based on both α-[V₁₄Sb₈O₄₂]⁴⁻ and β-[V₁₄Sb₈O₄₂]⁴⁻ isomers has been synthesized.

High Nuclearity Antimonato-Polyoxovanadate Cluster {V15 Sb6 O42 } as a Synthon for the Solvothermal in situ Generation of α- and β-{V14 Sb8 O42 } Isomers

New heteroatom polyoxovanadates (POVs) were synthesized by applying a water-soluble high-nuclearity cluster as new synthon. The [V15 Sb6 O42 ](6-) cluster shell exhibiting D3 symmetry was in situ transformed into completely different cluster shells, namely, the α-[V14 Sb8 O42 ](4-) isomer with D2d and the β-[V14 Sb8 O42 ](4-) isomer with D2h symmetry. The solvothermal reaction of {Ni(en)3 }3 [V15 Sb6 O42 (H2 O)x ]⋅15 H2 O (x=0 or 1; en=ethylenediamine) in water led to the crystallization of [{Ni(en)2 }2 V14 Sb8 O42 ]⋅5.5 H2 O containing the β-isomer. The addition of [Ni(phen)3 ](ClO4 )2 ⋅0.5 H2 O (phen=1,10-phenanthroline) to the reaction slurry gave the new compound {Ni(phen)3 }2 [V14 Sb8 O42 ]⋅phen⋅12 H2 O with the α-isomer. Both transformation reactions are complex due the change of symmetry, the chemical composition, and rearrangement of the VO5 square pyramids and Sb2 O5 handle-like moieties.

Vanadoantimonates: from discrete clusters to high dimensional aggregates

Five new vanadoantimonates have been obtained under hydrothermal conditions and characterized.